To protect public health and the environment, the United States Environmental Protection Agency Clean Air Highway Diesel final rule stipulated a 97% reduction in sulfur content of highway diesel fuel beginning in June 2006. Accordingly, diesel fuel, for use in locomotives, marine vessels, and in automotive vehicles, was altered so that the sulfur content was reduced from 500 parts per million (ppm) in low sulfur diesel to 15 ppm, referred to as ultra-low sulfur diesel.
From as early as 2007, the Petroleum Equipment Institute started receiving reports of unusually severe and accelerated corrosion of both fiberglass and metal parts associated with storage tanks and equipment used for dispensing, transporting, and storing ultra-low sulfur diesel. Reports included, for example, observations of a metallic coffee ground type substance clogging the dispenser filters and of corrosion and/or malfunctioning of seals, gaskets, tanks, meters, leak detectors, solenoid valves, and riser pipes.
The presence of acetic acid in high concentration in the vapor sampled from various ultra-low sulfur diesel containing tanks, as well as the concentration of acetate in the water bottoms, suggest that acetic acid may be reacting with the iron to produce the scale and corrosion observed of the corroded equipment. As such, it is believed that the corrosion ultimately arises from increased levels of acetic acid in the ultra-low sulfur diesel, where the acetic acid is likely being produced by acetic acid producing bacteria feeding on low levels of ethanol contamination, possibly by the following reaction:C2H5OH+O2→CH3COOH+H2OIt was found that fuel comprising even as little as 0.0033 volume percent of ethanol in the presence of enough bacteria and oxygen could result in high enough amounts of acetic acid to cause extensive corrosion.
Acetic acid producing bacteria is likely to be the cause of the increased levels of acetic acid as bacteria of the family Acetobacteraceae was found to be present in the bottom water and/or in the sediment that accumulates in, for example, the storage tanks. Bacteria of the family Acetobacteraceae, specifically of the genus Acetobacter, are known to metabolize ethanol into acetic acid in the presence of oxygen and water in slightly acidic conditions. It is believed that higher levels of acetic acid producing bacteria are present in ultra-low sulfur diesel as compared to low sulfur diesel due to the higher levels of sulfur functioning as a natural biocide in the low sulfur diesel.
A system and method for reducing the amount of acetic acid producing bacteria is therefore desirable in order to reduce the levels of acetic acid and to ultimately reduce the amount of corrosion of the equipment used in the storage and transport of fuel.